1. Field Of The Invention
This invention relates to a process for the separation of saccharides by molecular exclusion chromatography to reduce the amount of eluting solvent used in the separation.
2Description Of the Prior Art
There is a large market for starch conversion products having bland taste, low sweetness and low hydroscopicity. Such products are useful as bases for the preparation of food items as well as bodying agents and additives having non-sweet, water-holding, non-hygroscopic characteristics. Such products can also be used as carriers for synthetic sweeteners, as flavor enhancing materials, as carriers for coloring agents, as a spray-drying adjunct for coffee extract and for tea extract, as a bulking, bodying or dispersing agent in synthetic creams or coffee whiteners, as moisture holding agents in breads, pastry, and meats and as bodying and smoothing agents in puddings, fruits and frozen ice deserts.
In general, the relative sweetness of a starch conversion product depends upon the dextrose equivalent value, D.E. of the product. The term "D.E." as used herein refers to the reducing sugars content of the dissolved solids in a starch conversion product, expressed as percent dextrose as measured by the Schoorl method described in detail in Encyclopedia Of Industrial Chemical Analysis, Volume 11, pp. 41-2. In many applications, it is desirable to utilize a starch conversion product which has a low D.E. as well as one which exhibits extreme clarity and will not develop haze upon standing.
Starch conversion products are generally produced by the hydrolysis of starch in accordance with methods now known to those skilled in the art. For example, starch may be hydrolyzed by the use of a strong acid whereby the acid serves to cleave the dextrose units forming the starch molecule. Acid hydrolysis is generally a random process, and produces a hydrolysate containing starch fragments of many varying lengths, ranging from dextrose up to high molecular weight fragments formed by only slight hydrolysis of the starch. The starch fragments produced by hydrolysis are generally identified in terms of their degree of polymerization, DP. The term DP.sub.1 refers to a starch hydrolysis fragment having a degree of polymerization of 1, that is dextrose, whereas a DP of 2 refers to a starch fragment having a degree of polymerization of 2, containing 2 dextrose units (maltose) and a DP of 3 refers to a fragment containing 3 dextrose units (maltotriose).
Starch can also be hydrolyzed through the use of a number of enzymes which likewise cleave the starch molecule at the oxygen linkage between the repeating dextrose units. Enzymatic hydrolysis, too, results in the formation of a somewhat more specific and narrower distribution of molecular weight fragments as compared to acid hydrolysis.
Various improvements in the acid and/or enzymatic hydrolysis of starch have been made in the art. For example, it has been found that the specificity of the starch hydrolysis reaction can be more accurately controlled through the use of a two-step enzymatic hydrolysis as described in U.S. Pat. No. 3,853,706. In the process described in that patent, it is shown that the amount of DP.sub.1, DP.sub.2 and DP.sub.3 formed in an enzymatic hydrolysis is more limited when a two-step enzymatic process is employed. Another process to provide more specific control in the degrees of polymerization is described in U.S. Pat. No. 3,560,343 in which a two-step hydrolysis is carried out on starch, with the first step using an acid and the second step being carried out with an enzyme.
No matter how the starch hydrolysates are produced, there is invariably formed a distribution of polysaccharides. It is frequently desirable to provide starch hydrolysates which are rich in certain polysaccharide fractions. For example, high maltose syrups, that is, syrups containing a higher proportion of DP.sub.2 than would otherwise be produced by acid and/or enzymatic hydrolysis of starch, have found use in certain applications such as in the manufacture of hard candies and in coating of cereals.
Up to the present, no hydrolysis method has been discovered in which certain desired distribution of saccharides can be economically obtained. Thus, there is a need to provide a method for efficiently and inexpensively separating saccharides each from the other to alter the saccharide distribution of complex polysaccharide mixtures.
A somewhat similar problem exists with respect to levulose, i.e., fruit sugar. It is now common practice to produce levulose by first hydrolyzing corn starch to produce a high dextrose syrup. That syrup is then treated with a xylose isomerase enzyme to convert the dextrose to its ketose counterpart, levulose. Since the starting material for the isomerization is produced by hydrolysis of starch, both the starting material for the enzymatic isomerization as well as the isomerate produced, contains polysaccharides as impurities in the conversion product containing mostly dextrose and levulose. It is therefore desirable to provide a simple and inexpensive means for separating levulose from dextrose and polysaccharides to produce a high levulose-bearing syrup.
The prior art, as represented by U.S. Pat. Nos. 3,756,919, 3,785,864 and 3,806,363, has suggested the use of exclusion chromatography to separate polysaccharides each from the other and to separate dextrose from levulose. As is now well known to those skilled in the art, exclusion chromatography involves the use of a chromatography column of bed which is packed with a porous adsorbent. The feed or supply to the column, usually in the form of a solution, is passed through the column followed by an eluting solvent. Various fractions are removed from the bottom of the column or bed, with the higher molecular weight materials which are present in the feed coming out of the column first followed by lower molecular weight components. As is now well understood by those skilled in the art, such higher molecular weight components of the feed mixture do not diffuse as rapidly into the pores of the chromatography packing material, and thus pass through the column more quickly than lower molecular weight material which tend to diffuse into the pores of the column packing. The eluting solvent serves to wash the materials from the packing of the column, the rates of elution depending upon the molecular weight and chemical structure of the various components.
In the separation of polysaccharides and in the separation of levulose from dextrose and polysaccharides, it has been found preferably to employ water as the eluting solvent. It is readily available at nominal cost, and it does not result in contamination of either the column or the components to be separated therein. However, to assure separations as completely as possible, it is necessary to employ large quantities of water as the eluting solvent. As a result, each of the fractions removed from the column becomes diluted. Since starch hydrolysates as well as levulose are used in the form of either a concentrated syrup or dried solids, a large proportion of the water introduced to the column must be removed through evaporation to produce a product ready for use. The concentration of the dilute exclusion products has thus been one of the significant economic disadvantages in the use of exclusion chromatography to separate polysaccharides each from the other and the separation of levulose from dextrose and polysaccharides.